Can't Convert to Curve|Mesh using ALT+C in linux
Shortcuts on the OS take precedence over those in blender.
There are two options:

You can remap the shortcuts on linux to the super (windows) key.
Read: How to use the Alt + Right click in edit mode in Linux

You can customize the shortcuts on blender, just know that any books or &quot;tutorials&quot; you watch will not match with your personalized keyboards.